1. Field of the Invention
The present invention relates to a communication system in which a plurality of subscriber units (ONU's) are connected to a network unit (OLT) through a common medium and the subscriber units transmit information by polling information from the network unit such as an ATM-PON, and in particular to an allocation control of transmission grant information used for the polling information.
2. Description of the Related Art
1. Frame Structure of ATM-PON
The specification of an ATM-PON system is prescribed by the ITU-T recommendation G.983.1at present. FIG. 20 shows a frame structure in an ATM-PON transmission system prescribed by the G.983.1. A single frame is composed of 23744 bits at 155.52 Mb/s in both of a down direction from a network unit (OLT) to a subscriber unit (ONU) and an up direction from the subscriber unit to the network unit.
In the down direction, 53-byte cells are transmitted in series. Two transmission rates of 155.52 Mb/s and 622.08 Mb/s are prescribed, so that 56 cells compose a single frame for the 155.52 Mb/s and 224 cells compose the same for the 622.08 Mb/s. On the other hand, the up direction adopts a transmission system where a plurality of subscriber units intermittently transmit 56-byte cells. The transmission rate is 155.52 Mb/s only, so that 53 slots of 56-byte length-time slots compose a single virtual frame.
The cells in the down direction are broadly divided into physical layer OAM (hereinafter, referred to as PLOAM) cells and ATM cells. The PLOAM cell is always transmitted at the head of a block when a frame is divided into 28-cell blocks (2 blocks for 155.52 Mb/s, and 8 blocks for 622.08 Mb/s). FIG. 21 shows a structure of a PLOAM cell transmitted in the down direction.
On the other hand, the cells in the up direction include a cell (mini cell) shorter than a 53-byte cell in addition to the two kinds of the down cells. This mini cell is transmitted by a slot (Divided_slot) obtained by further dividing a 56-byte time slot into plural slots. Which subscriber unit transmits a cell, which cell is transmitted, and to which time slot the cell is transmitted are designated by the network unit with a method described later. FIG. 22 shows the structure of an up direction PLOAM cell.
2. Transmission Control of up Direction Cell
Hereinafter, a designation method for the above-mentioned up direction cell transmission will be described.
Firstly, in order for the subscriber unit to transmit the cell, the network unit is required to allocate a number for each cell transmission grant. This transmission grant number (grant) is prescribed as ATM cell transmission grant information (Data grant), PLOAM cell transmission grant information (PLOAM grant), or mini cell transmission grant information (Divided_slot grant), according to the classification of the cell.
The cell transmission grant information and the PLOAM cell transmission grant information are individually allocated to each of the network units, and the mini cell transmission grant information is individually allocated to each group composed of plural network units, respectively without being mutually superimposed. The allocated information is notified to the concerned subscriber unit by a message.
It is to be noted that the “message” is control information or the like between the network unit and the subscriber unit, and is transmitted by the PLOAM cell. The region of “MESSAGE_**” within each PLOAM cell is a region for transmitting an up/down message, and a message for a transmission grant notification is also prescribed within the down message.
As a message for notifying the allocation of the data cell transmission grant information and the PLOAM cell transmission grant information, a transmission grant allocation (Grant_allocation) message is prescribed; As a message for notifying the allocation of the mini cell transmission grant information, a transmission grant setting (Divided_slot_grant_configuration) message is prescribed; Those messages will be described later. As for the mini cell transmission grant information, various setting information (payload length, transmission start position, and service ID) of a mini cell transmitted to the mini cell transmission grant information is also notified to the subscriber unit.
If the transmission grant information is allocated to each of the subscriber units, an actual designation of a cell transmission is performed in a region of a “GRANT**” in the PLOAM cell.
The region “GRANT**”, as shown in FIG. 23, corresponds to each of the time slots of the up frame depending on how manieth PLOAM cell it is in the down frame. Regardless of the down transmission rate, the regions from the first region GRANT1 of the head PLOAM cell to the region GRANT26 of the second PLOAM cell in the down frame are sequentially designated to the time slots of the up frame from the head time slot. The network unit writes various transmission grant information in each region GRANT** to be notified to each subscriber unit, while the subscriber unit transmits various cells to the corresponding position of the up slot when finding the transmission grant information notified by the above-mentioned message in the region.
3. Message used for Allocating Transmission Grant Information
A message conveys controlling information or the like between the network unit and the subscriber unit, and is transmitted in the region indicated by MESSAGE_** of the up and the down PLOAM cells. Various messages are prescribed for the up/down cells respectively. Among these messages, the following transmission grant allocation message and a mini cell transmission grant setting message are prescribed in the down direction message as is used for allocating the transmission grant information.
4. Transmission Grant Allocation Message (See FIG. 24).
This message serves to notify the codes of the data cell 10 transmission grant information and the PLOAM cell transmission grant information, and their validity/invalidity (activation/deactivation) settings to the subscriber unit.
Upon granting the transmission of the above-mentioned cell to the subscriber unit, the network unit firstly notifies, by this message, the code of the transmission grant information and that it is valid (active).
The subscriber unit processes the message if its destination is the subscriber unit itself, and performs setting the number to which the subscriber unit itself should transmit the PLOAM cell or ATM cell otherwise if the information is valid. If the information is invalid (deactive), the subscriber unit stops the cell transmission with respect to the number.
At present, the message is not prescribed to perform a processing in an operation state, in which an allocation and a validity/invalidity switchover are not prescribed.
5. Mini Cell Transmission Grant Setting Message (See FIG. 25)
This message notifies the number of the mini cell transmission grant information, and the validity/invalidity thereof, as well as the information related to the position and the length to the subscriber units.
The subscriber units performs the same validity/invalidity processing as the transmission grant allocation message (see FIG. 24) to the notified mini cell transmission grant information if the destination is the subscriber unit itself. Also, as for this transmission grant information, the subscriber unit sets/releases the payload length, an offset value, a service ID of the mini cell transmitted to the transmission grant information.
FIG. 26 shows a relationship between a payload length (LENGTH), a head position (OFFSET), and an up time slot (Divided_slot). At present, the allocation of the mini cell transmission grant information is not prescribed in detail.
FIG. 27 shows a basic arrangement of the prior art communication system by the above-mentioned ATM-PON transmission system.
In the network unit OLT, a transmission grant information manager manages various transmission grant information and setting of validity/invalidity on the subscriber units ONU's one-on-one, and manages the payload length of the cell of the subscriber unit ONU top which the mini cell transmission grant information is allocated, an offset value indicating the position in the slot, and a service ID as for the mini cell transmission grant information.
When changing the setting, a message generator 2 generates a message by which the setting change is notified to the subscriber unit ONU prescribed times (three times) according to the request from the transmission grant information manager 1.
A polling information generator 3 generates polling information by using transmission grant information set as being “valid” in the transmission grant information manager 1, and notifies the generated polling information to an input disconnection detector 4.
The generated massage and polling information are mapped into the PLOAM cell inserted at 28-cell intervals by a multiplexer 5 to be transmitted to the subscriber unit ONU.
The input disconnection detector 4, based on the information generated at the polling information generator 3, manages a source subscriber unit ONU of an arriving cell, monitors the input of the cell for every subscriber unit ONU, and detects an input disconnection state (LOSi) in the absence of an 8-time continuous valid input for the subscriber unit ONU.
An identifier 6 identifies a received cell according to a transmission grant information classification for each subscriber unit to be processed.
In the subscriber unit ONU, a message processor 21 acquires the various transmission grant information form the PLOAM cell extracted at the PLOAM cell demultiplexer 22, and sets the validity/invalidity thereof. As for the mini cell transmission grant information, the message processor 21 further extracts the payload length and the offset value.
A transmission grant information manager 23 manages the various transmission grant information one-on-one in accordance with the setting contents extracted by the message processor 21. As for the mini cell transmission grant information, the transmission grant information manager 23 also manages the various setting information (payload length, offset, service ID).
A polling information extractor 24 monitors the polling information transmitted by the PLOAM cell. When the various transmission grant information set valid is extracted at the transmission grant information manager 23, the polling information extractor 24 notifies a multiplexer 25 of the up information, the PLOAM cell, or the transmission phase of the mini slot if the transmission grant information is the cell transmission grant information, the PLOAM cell transmission grant information, or the mini cell transmission grant information, respectively.
The multiplexer 25 transmits various cells depending on the kind of cell and the transmission phase to be transmitted from the polling information extractor 24. When the PLOAM cell is transmitted, the message generated at a massage generator 26 is mapped.
In such a prior art communication system mentioned above, the present G.983.1 only prescribes that the subscriber unit processes all of the messages within a term of 6 frames at the maximum. This similarly applies to the above two messages. The subscriber unit may start (if valid; or stop if invalid) the transmission of the cell to the transmission grant information at any time if only within the term of 6 frames after receiving the messages.
Since the network unit can not recognize this timing, it starts the polling just after the message transmission by using the transmission grant information. The subscriber unit does not transmit the cell with respect to the polling before the processing is completed in the subscriber unit.
Furthermore, when no valid cell is inputted with respect to the polling prescribed times (8 times) continuously in the G.983.1, it is found to be the input disconnection state (LOSi), so that the processing is performed to make the corresponding subscriber unit the non-operation state. Therefore, if the subscriber unit satisfying the prescription has no consideration therefor, the non-operation processing is performed with the worst timing.
Also, if this point is not considered even in the startup, a cell is not transmitted in response to the transmission request of the cell, whereby the subscriber unit fails to start up successfully.
Also, as for the mini cell transmission grant information, a single transmission grant information is used for a group. Therefore, when a certain subscriber unit is not found valid/invalid, there are some cases where polling information by the transmission grant information is generated to the other subscriber unit in which the transmission grant information is valid, and there is a possibility that the input disconnection state is similarly detected.
Inherently, this is not a proper input disconnection detection. Also, upon the detection of the input disconnection state, the network unit performs a processing of making the subscriber unit the non-operation state when it is in the operation state. Therefore, some control is required for this processing.
Also, control is required for the case where a plurality of allocations are performed to the same kind of transmission grant information or different settings are instructed to the same transmission grant information as for the mini cell transmission grant information.